A German historian has claimed in a new book presented on Monday that Nazi scientists successfully tested a tactical nuclear weapon in the last months of World War II.

Rainer Karlsch said that new research in Soviet and also Western archives, along with measurements carried out at one of the test sites, provided evidence for the existence of the weapon.

"The important thing in my book is the finding that the Germans had an atomic reactor near Berlin which was running for a short while, perhaps some days or weeks," he told the BBC.

"The second important finding was the atomic tests carried out in Thuringia and on the Baltic Sea."

Mr Karlsch describes what the Germans had as a "hybrid tactical nuclear weapon" much smaller than those dropped on Hiroshima or Nagasaki.

'Bright light'

He said the last test, carried out in Thuringia on 3 March 1945, destroyed an area of about 500 sq m, killing several hundred prisoners of war and concentration camp inmates.

The weapons were never used because they were not yet ready for mass production. There were also problems with delivery and detonation systems.

Karlsch has done us a service in showing that German research into uranium went further than we'd thought... but there was not a German atom bomb

Michael Schaaf, German physicist

"We haven't heard about this before because only small groups of scientists were involved, and a lot of the documents were classified after they were captured by the Allies," said Karlsch.

"I found documents in Russian and Western archives, as well as in private German ones."

One of these is a memo from a Russian spy, brought to the attention of Stalin just days after the last test. It cites "reliable sources" as reporting "two huge explosions" on the night of 3 March.

Karlsch also cites German eyewitnesses as reporting light so bright that for a second it was possible to read a newspaper, accompanied by a sudden blast of wind.

The eyewitnesses, who were interviewed on the subject by the East German authorities in the early 1960s, also said they suffered nose-bleeds, headaches, and nausea for days afterwards.

Karlsch also pointed to measurements carried out recently at the test site that found radioactive isotopes.

Sceptical response

His book has provoked huge interest in Germany, but also scepticism.

It has been common knowledge for decades that the Nazis carried out atomic experiments, but it has been widely believed they were far from developing an atomic bomb.

The bomb was much smaller than the weapon dropped on Hiroshima

"The eyewitnesses he puts forward are either unreliable or they are not reporting first-hand information; allegedly key documents can be interpreted in various ways," said the influential news weekly Der Spiegel.

"Karlsch displays a catastrophic lack of understanding of physics," wrote physicist Michael Schaaf, author of a previous book about Nazi atomic experiments, in the Berliner Zeitung newspaper.

"Karlsch has done us a service in showing that German research into uranium went further than we'd thought up till now, but there was not a German atom bomb," he added.

It has also been pointed out that the United States employed thousands of scientists and invested billions of dollars in the Manhattan Project, while Germany's "dirty bomb" was allegedly the work of a few dozen top scientists who wanted to change the course of the war.

Karlsch himself acknowledged that he lacked absolute proof for his claims, and said he hoped his book would provoke further research.

But in a press statement for the book launch, he is defiant.

"It's clear there was no master plan for developing atom bombs. But it's also clear the Germans were the first to make atomic energy useable, and that at the end of this development was a successful test of a tactical nuclear weapon."